Crystal structure of [tBuMgCl]2[MgCl2(Et2O)2]2

The crystal structure of [tBuMgCl]2[MgCl2(Et2O)2]2 features an Mg4Cl6 open-cube cluster with both four- and six-coordinate Mg2+ ions. The Cl− ions adopt bridging positions. Intermolecular C—H⋯Cl hydrogen bonds link adjacent units of the title compound into chains extending parallel to [010].


Chemical context
Grignard reagents (RMgX) are among the most commonly used organometallic reagents in synthesis. However, their molecular structures are highly diverse and therefore subject to ongoing research (Elschenbroich, 2008;Peltzer et al., 2020;Curtis et al., 2020). The structures of RMgX in solution depend on the nature of the solvent, the bulkiness of the organic moiety R, and the choice of the halide X (Peltzer et al., 2017). Moreover, the Schlenk equilibrium can convert RMgX into MgR 2 and MgX 2 (Schlenk & Schlenk jun., 1929). The formation of halide bridges between the Lewis-acidic Mg 2+ ions (Mg-X-Mg) allows for dimeric and oligomeric structures that are in equilibrium with their monomeric units (Fig. 1). Further coordination sites at Mg 2+ are often saturated by donor-solvent molecules (Seyferth, 2009).
Since the analysis of Grignard reagents in solution is challenging, X-ray crystallography has emerged as an alternative, frequently used method to investigate their molecular compositions. A recurring structural motif in the solid state is the open-cube cluster [RMgCl(THF)] 2 [MgCl 2 (THF) 2 ] 2 (I; R = Me, Et, i Pr, n Bu, t Bu). Toney & Stucky (1971), Sakamoto et al. (2001), as well as our group (Blasberg et al., 2012) provided evidence for such structures obtained by crystallization of RMgCl from THF. According to the Schlenk equilibrium, the formation of I can be rationalized by assuming aggregation of two RMgClÁMgCl 2 entities. The two independent Mg 2+ ions in I exhibit five-and six-coordination, respectively. We now report [ t BuMgCl] 2 [MgCl 2 (Et 2 O) 2 ] 2 (II) as the first example of such open-cube clusters obtained from Et 2 O. It is noteworthy that, unlike those in I, the reactive Mg 2+ ions in the title compound II are four-coordinate and, surprisingly, no solvent coordinates to these t BuMgCl 3 entities. Subtle changes such as exchanging THF for the weaker donor Et 2 O seem to have a significant effect on the observed structural motifs (Fig. 2).

Supramolecular features
There are two short C-HÁ Á ÁCl contacts bridging adjacent molecules of the title compound II. These hydrogen bonds (Table 1)  Open-cube structures like I (R = t Bu) are obtained by crystallization of t BuMgCl from THF solutions, whereas the less-solvated title compound II crystallizes from Et 2 O solutions of t BuMgCl.

Figure 3
Molecular structure of the title compound. Displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms are omitted for clarity.  Casellato & Ossola, 1994). A seventh structure [ n BuMg 3 Cl 5 (THF) 4 ] 2 (ZIHQEO; Pirinen et al., 2013) also features an open-cube cluster; however, here the reactive Mg 2+ ions are not part of the cubes. The latter structure is therefore not included in the comparison. Interestingly, all the above structures from the database show crystallographic centrosymmetry, with all of them being located at a center of inversion. The title compound, on the other hand, does not show any crystallographic symmetry, although it would be possible for II to comply with a crystallographic inversion center. A fundamental difference between the structure of the title compound and the published struc-tures is the coordination sphere of the reactive Mg 2+ ions. In all structures retrieved from the CSD, these Mg 2+ ions are fivecoordinate and the ligands form a distorted trigonal bipyramid. The calculated geometry indices 5 (0.65-0.84) show a varying degree of deviation from the ideal trigonal bipyramidal geometry ( 5 = 1; Addison et al., 1984). The MgÁ Á ÁCl distances to the 3 -Cl À ligands in the central Mg 2 Cl 2 plane (Table 2)  Packing diagram of the title compound II showing the C-HÁ Á ÁCl hydrogen bonds (cyan) between adjacent molecules of II. H atoms not involved in hydrogen bonding are omitted for clarity. Table 2 Comparison of MgÁ Á ÁCl distances (Å ) and sums of equatorial angles AE eq ( ) of the five-coordinate Mg 2+ ions in the literature phases.

Database survey
There are two rows for the title compound because it does not show any symmetry, whereas all structures retrieved from the database are located at a center of inversion. MgÁ Á Á 2 -Cl: bond lengths are between the five-coordinate Mg 2+ ions and the 2 -Cl À ions. MgÁ Á Á 3 -Cl: bond lengths are between the fivecoordinate Mg 2+ ion and the 3 -Cl À ion in the central Mg 2 Cl 2 plane.

Synthesis and crystallization
Magnesium turnings (9.74 g, 401 mmol, 1.7 eq.) were placed in a Schlenk flask and dried under vacuum by heating.

Di-µ 3 -chlorido-tetra-µ 2 -chlorido-tetrakis(diethyl ether-κO)bis(1,1-dimethylethyl)tetramagnesium
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.